1. Field of the Invention
The present invention relates to a grinding machine and to a method for grinding semiconductor bars.
2. The Prior Art
Monocrystalline semiconductor bars which have been produced using the Czachralski crucible drawing method have an irregular surface and a fluctuating length. The irregular surface means that the diameter fluctuates along the semiconductor bar and decreases in some places and increases again at other places. This may even lead to a "corkscrew" shape. The result is that the bar has to be ground down to the specified desired diameter, so that the bar has a uniform diameter before it is cut into wafers.
Usually, the semiconductor bars are ground using a plunge-grinding wheel or cup wheel. The plunge-grinding wheel is a grinding wheel in which the width of the circumferential surface of the wheel is used for grinding. A cup wheel is a grinding wheel which is a straight or conical, circular grinding body with, on one side, a deep cutout and hole, in which the thickness of the base or of the web is less than 1/3 of the total width. That is, when its attachment means is facing downward, its top edge forms the grinding element.
For conventional rough and precision grinding using a plunge-grinding wheel, this wheel is considerably deprofiled on the left-hand edge or right-hand edge during the finishing precision grinding or oscillation grinding. This deprofiling considerably reduces the effective width of the abrasive coating. This takes place because the semiconductor bar is moved past the plunge-grinding wheel with a lateral feed. This means that the plunge-grinding wheel is subject to considerable abrasion in particular at its flanks. The drawback of this is that the plunge-grinding attachment has to be corrected relatively often. Thus the plunge-grinding wheel has to be reground and resurfaced frequently in order for the full grinding width to be available again. This has the result that valuable diamond coating is lost without producing any benefit. This leads to higher grinding material costs. A further drawback which results from the regrinding and resurfacing is an increased cost factor, since during this period the machine and the staff are not available for production.
Furthermore, grinding machines of this nature are not tolerant of flaws. This means that before grinding it is always necessary to input the appropriate diameter. Input errors may occur, resulting in too much material being ground off the semiconductor bar, making this bar unusable. Furthermore, incorrect input of this nature may cause the wheel to be advanced too far, thus moving into the semiconductor bar and damaging the bar and, under certain circumstances, damaging the grinding machine as well.
When grinding using the cup wheel, the semiconductor bar, which is guided past the rotating cup wheel with a lateral feed, is ground carefully. A drawback of this method compared to grinding using the plunge-grinding wheel is that this form of grinding requires approximately twice as much time to grind a bar as does grinding using the plunge-grinding wheel.